Realtime Anaglyphic viewport
This feature was added to Blender in version 2.75a.

To enable it, you have to be in Camera View, which you can enable by pressing Numpad 0. Optionally, you can lock the camera to your view by pressing selecting View > Lock Camera to View in the panel on the right-hand side of the 3D View (N).

To enable the Anaglyphic viewport, go to the Render Layers tab of the Properties panel, and check the Views checkbox. The default settings should be sufficient:



This is what it looks like:



For more detailed information, check out the relevant Blender Manual page.